@Sotirios Delimanolis answer do the job however I was looking for comparing strings within this mockMvc assertion
So here it is
.andExpect(content().string("\"Username already taken - please try with different username\""));
Of course my assertion fail:
java.lang.AssertionError: Response content expected:
<"Username already taken - please try with different username"> but was:<"Something gone wrong">
because:
MockHttpServletResponse:
Body = "Something gone wrong"
So this is proof that it works!